Lesson 5 Controlling LED by Button

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Introduction

In this lesson, we will learn how to turn an LED on or off by a button.

Newly AddedComponents

Principle

Button

Button is a common component used to control electronic devices. It is usually used as switch to connect or break circuits. Although buttons come in a variety of sizes and shapes, the one used here is a 6mm mini-button as shown in the following pictures.

Two pins on the same side are connected, which is shown below:

This following symbol represents a button in circuit.

When the button is pressed, the 4 pins are connected, thus closing the circuit.

Schematic Diagram

When the button is pressed once, pin 32 is 3.3V (HIGH). Set the pin 11(integrated with an LED) as high level by programming at the same time. Then press the button again and set pin 11 to Low. So we will see the LED light on and off alternately as the button is pressed many times.

Build the Circuit

  • For C Language Users

Command

1.Go to the folder of the code.

cd /home/pi/electronic-kit/for-raspberry-pi/c/Lesson_5_Controlling_Led_by_Button

2. Compile the code.

gcc 5_Button.c -lwiringPi 

3.Run the executable file.

sudo ./a.out

When you press the button for the first time, the LED lights up. When the button is pressed again, the LED lights off.

Code

1.#include <wiringPi.h>  
2.#include <stdio.h>  
3.  
4.#define LedPin      0  
5.#define ButtonPin   26  
6.int state = 0;  
7.  
8.int main(void){  
9.    // When initialize wiring failed, print message to screen  
10.    if(wiringPiSetup() == -1){  
11.        printf("setup wiringPi failed !");  
12.        return 1;   
13.    }  
14.      
15.    pinMode(LedPin, OUTPUT);   
16.    pinMode(ButtonPin, INPUT);  
17.    pullUpDnControl(ButtonPin, PUD_DOWN);  
18.  
19.    while(1){  
20.        // Indicate that button has pressed down  
21.        if(digitalRead(ButtonPin) == 1)  
22.        {  
23.            delay(10);  
24.            if(digitalRead(ButtonPin) == 1)  
25.            {  
26.                state ++;  
27.                if(state%2 == 1)  
28.                {  
29.                    digitalWrite(LedPin,HIGH);  
30.                    delay(100);  
31.                }  
32.                if(state%2 == 0)  
33.                {  
34.                    digitalWrite(LedPin,LOW);  
35.                    delay(100);  
36.                }  
37.            }  
38.        }         
39.    }  
40.    return 0;  
41.}  

Code Explanation

6.int state = 0;

Define a variable state to record the number of times it is pressed and the initial number of times is 0.

15.  pinMode(LedPin, OUTPUT);

16.    pinMode(ButtonPin, INPUT);

Set the LedPin to OUTPUT mode, ButtonPin to INPUT mode.

17.pullUpDnControl(ButtonPin, PUD_DOWN);

When the button is not pressed, ButtonPin is in suspension at which time the read value is changing. To enable ButtonPin to output a stable low level, PUD_DOWN is added to the code, keeping ButtonPin at the forced pull-down state till the button is pressed.

21.        if(digitalRead(ButtonPin) == 1)

22.        { 

23.            delay(10);

24.            if(digitalRead(ButtonPin) == 1)

25.            {

Usually the buttons we use are mechanical buttons, so in the process of pressing down and releasing, there will be no direct change from 0 to 1, but will be more than 10ms of level jitter. In order to ensure that the program only responds to the button once when it is closed or broken, the jitter elimination of the button must be carried out. An if function is used to detect whether the button is pressed. When the signal of the button is pressed is detected, a delay of 10ms is used to eliminate the possibility of false judgment, and another if function is used to detect again. If both if conditions are met, confirm that it is a button press, and then execute the program in the if.

26.                state ++;

If the button is pressed, the number of times it is pressed is increased by one. (state ++ is the same as state = state+1).

32.                if(state%2 == 0)

33.                {

34.                    digitalWrite(LedPin,LOW);

35.                    delay(100);

36.                    delay(100);

Here, judge whether state%2 is equal to 0. If so, it means that the number of times the button is pressed is an even number, and then turn off the LED.

  • For Python Language Users

Command

1.Go to the folder of the code.

cd /home/pi/electronic-kit/for-raspberry-pi/python

2. Run the code.

sudo python3 5_Button.py

When you press the button for the first time, the LED lights up. When the button is pressed again, the LED lights off.

Code

1.import RPi.GPIO as GPIO  
2.import time  
3.  
4.LedPin = 17  
5.BtnPin = 12  
6.Led_status = False  
7.  
8.# Define a setup function for some setup  
9.def setup():  
10.    GPIO.setmode(GPIO.BCM)  
11.    GPIO.setup(BtnPin, GPIO.IN)  
12.    GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.LOW)  
13.    GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed)  
14.  
15.# Define a callback function for button callback  
16.def swLed(ev=None):  
17.    global Led_status  
18.    Led_status = not Led_status  
19.    GPIO.output(LedPin, Led_status)  
20.  
21.# Define a main function for main process  
22.def main():  
23.    while True:  
24.        # Don't do anything.  
25.        time.sleep(1)  
26.  
27.# Define a destroy function for clean up everything after  
28.# the script finished   
29.def destroy():  
30.    # Turn off LED  
31.    GPIO.output(LedPin, GPIO.LOW)  
32.    # Release resource  
33.    GPIO.cleanup()  
34.  
35.# If run this script directly, do:  
36.if __name__ == '__main__':  
37.    setup()  
38.    try:  
39.        main()  
40.    # When 'Ctrl+C' is pressed, the child program   
41.    # destroy() will be  executed.  
42.    except KeyboardInterrupt:  
43.        destroy() 

Code Explanation

6.Led_status = False

Set a variable Led_status to record the current status of the LED; when Led_status is True, it indicates that the current lamp is in bright state; when Led_status is False, it means that the light is off.

7.    GPIO.setup(BtnPin, GPIO.IN)

Set BtnPin as input mode to read the state of the button to determine whether to execute the corresponding program. Note that when GPIO.setup sets the pin to input mode, then there is no need to set the initial value.

12.  GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.LOW)

Specify an initial value for your output channel. Here the LED is the output component, so we set LedPin to GPIO.OUT mode. Then initialize the state of LED to GPIO.LOW which means that the light is off.

13.    GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed)

The event_detected() function is designed to be used in a loop with other things, but unlike polling it is not going to miss the change in state of an input while the CPU is busy working on other things. Set up a falling detect on BtnPin, when the BtnPin pin is detected to change from high level to low level, swLed function is called.

16.def swLed(ev=None):

17.    global Led_status

18.    Led_status = not Led_status

19.    GPIO.output(LedPin, Led_status)

RPi.GPIO runs a second thread for callback functions. This means that callback functions can be run at the same time as your main program, in immediate response to an edge. Define a callback function for button callback, execute the function after the callback of the interrupt. When this function is executed, the state of the LED is firstly reversed(If True, make it False, and vice versa). Then input the function to LedPin. And “ev = None” means that if no parameter is passed when calling swLed, take None as the default value of ev.